Previous studies have disclosed that, under the influence of elastic and plastic strains at the temperature below the point corresponding to the beginning of strain-induced martensite formation (Md), a number of metastable austenitic steels and alloys undergo phase transformations of g
type. The transformations result in the formation of stress-assisted or strain-induced martensite. Strain-induced martensite significantly affects the properties of steels: it improves the intensity of strain hardening, reinforces the uniform elongation capacity of the material and, under conditions of cooling the metal below Md, it may lead to unusual increase in elongation, which is referred to as the transformation plasticity effect . The presence of ferromagnetic a
-martensite even in the amount as low as 3-5% significantly reduces operating properties of metastable steels and alloys, e.g. corrosion resistance and magnetic permeability rate, thus a number of steels can no longer be used as paramagnetic materials in structures with high magnetic protection. As distinct from Fe-Ni-based alloys, the metastable steels and alloys based on Fe-Mn and Fe-Mn-Cr have not been so far examined in the course of loading within elastic deformation and plastic deformation areas. All the available data concerning the a
-strain-induced martensite content measurements were obtained by means of a Shteinberg-Zyuzin magnetometer with the measurement error within ±
1% on unloaded tensile test pieces, which had been strained to a certain degree, starting from 5%. The presence of the a
-phase can be explicitly determined by means of the X-ray diffraction analysis, but the uncertainty in the determination is of the order of 5%.
Materials and experimental procedures
A study was made on one Fe-Mn-based (steel grade 1) and two Fe-Mn-Cr-based (steel grades 2 and 3) strain-metastable steels with (g
)- dual-phase and austenitic structure. The chemical analysis and phase composition of these steels after initial water quenching from 1050°
C are presented in table 1. The phase composition and strain stability of the examined steels differed significantly due to variable amounts of such elements as carbon, nitrogen, silicon and chromium, which substantially affect the Mn and Md temperatures .
The amount of ferromagnetic phase in the examined steels was measured under the room temperature using a low-magnetic modification of a specially designed set-up. The set-up made it possible to load the test pieces under conditions of uniaxial tension under the tensile stress to as high as 50kN and to record stress-strain curves simultaneously. Besides, a built-in ferritometer (i.e. an a
-phase content estimating facility) made it possible to conduct direct measurements of the amount of ferromagnetic a
-phase within a test piece in the course of loading up to the initial point of local necking and further, up to the point of fracture.
* steel grade 2 as quenched contained ferromagnetic d-ferrite in the amount of 4%.
||Chemical composition, wt.%
||Phase composition ,%
|| 55 (e)
|Table 1: Chemical and phase composition of the examined steels|
Discussion of experimental results
Under the influence of elastic and plastic strains the examined Fe-Mn- and Fe-Mn-Cr-based steels with both austenitic and dual-phase structure consequently undergo g
martensitic transformations . The formation of a
-phase so far was observed only after significant plastic strain, e.g. above 5% elongation. The analysis has testified that the initial formation of a
-strain-induced martensite in the amount of 0,5 to 2,5% takes place within the macro-elastic region of the stress-strain curves, when the stress applied is approximately equal to the proof strength s
Fig 1: a
¢-Phase contents vs elongation|
The tensile tests have shown that the chemical and phase composition of the steels tested considerably influence the stress-strain curve profiles (fig. 1). It has been established that maximum strength is associated with nitrogen bearing steel 3, while the lower plasticity is observed in (g
)- dual-phase steel grade 1. However, all the steel grades have shown a very important common feature: all of them exhibit high capacity for homogeneous distribution (delocalization) of plastic flow, which provides highly uniform elongation of test pieces. Steel grade 1 based on Fe-Mn has shown the most intensive increase in the amount of ferromagnetic phase within the area of uniform straining up to elongation as high as 15%, the a
-phase formation rate remaining constant up to the point of necking (fig. 2). Austenitic Fe-Mn-Cr-based steel grades 2 and 3 exhibit not so steep dependence of a
-strain-induced martensite content upon the elongation degree. In all the steel grades examined the transition from the homogeneous mode of deformation to the localized one was accompanied by a dramatic rise in the amount of the ferromagnetic a
-phase. A combined analysis of the magnetic phase content measurements and of the true stress-strain curve obtained for the most stable steel grade 2 has revealed that a substantial increase in the amount of a
-martensite takes place within the necking area, the latter being associated with high three-axial tensile stresses and intensive plastic deformation.
Fig 2: Stress-strain curves|
It should be noted that the present experimental results have been obtained by direct measurements using precise magnetic technique and that they have exhibited high accuracy and reproducibility of data. Besides, they are in good agreement with previous results obtained by the magnetometric method involving a number of consequent loading-unloading cycles. Some extra magnetic phase content measurements performed on unloaded test pieces have not revealed any alterations in phase composition, which could testify to a drop in the a
-martensite content associated with possible reversibility of g
- The formation of magnetic a
-strain-induced martensite, viz. its initial precipitation and further increase in the amount, is observed within the macro-elastic region on the stress-strain curves, when the stress applied is approximately equal to the proof strength s
- A substantial increase in the amount of a
-phase takes place within the initial stages of deformation. As the deformation processes go further, the content of a
-phase remains constant or exhibits steady growth with the strain rate.
- The most dramatic rise in the amount of a
-strain-induced martensite in the course of mechanical loading is exhibited under localized deformation, particularly, at the point of local necking which is attributed to the change in the stress state mode resulting from the increase in tensile strain portion and intensive plastic deformation.
- The measurements of magnetic properties make it possible to apply non-destructive inspection methods for technical diagnostics of plastic strain in metastable austenitic steels.
- Fachr D. Stress- and strain-induced formation of martensite and effects on strength and ductility of metastable austenitic stainless steels. - Met. Trans. 2, 1971, p. 1883-1892.
- Bogachev I.N., Egolaev V.F. Structure and properties of ferromagnetic alloys. - Moscow, Metallurgia, 1973, 259 p. (in Russian)